1. Field of the Invention
The present invention relates to a magnetic sensor suitably applied to a read-out sensor which can read out a scale of a magnetic scale and to a position detector suitably applied to a machine tool or the like, for example.
2. Description of the Related Art
A position detector has been proposed so far, in which a magnetic sensor disposed in an opposing relation to a magnetic scale magnetized at a predetermined grating pitch is moved relatively with respect to the magnetic scale to thereby detect a relative position or the like from an electrical signal obtained on the basis of the change of a magnetic field. This position detector is applied to a machine tool which performs accurate machine work. In the industrial field utilizing the machine tool or the like, it is desired that the position detector becomes higher in accuracy when it is in use. In this case, a required resolution is about 0.1 .mu.m and an interpolation accuracy is about 0.8 .mu.m.
In order to realize the above-mentioned high accuracy, a grating pitch (i.e., recording wavelength) formed on the magnetic scale must be reduced to about 80 .mu.m, for example, and an electrical division (interpolation) must be carried out simultaneously.
When the above-mentioned conditions are satisfied, an output signal from a magnetic sensor must satisfy three requirements below:
(1) A waveform of an output signal should become a sine wave which is less in distortion. For example, when a pitch of the output signal is electrically divided by 400, then a sine waveform whose signal-to-noise (S/N) ratio is about 50 to 60 dB must be provided.
(2) The output signal has no fluctuation (hereinafter referred to as a dc shift, if necessary) of a dc component. If any, only an extremely small dc fluctuation is permitted. For example, in order to obtain an accuracy of about 1/100 of the grating pitch, the fluctuation of the dc component must be reduced to less than 1% of the amplitude.
(3) An amplitude fluctuation of an output signal should be either locally or wholly small.
A magnetic sensor which can satisfy the aforesaid three requirements must be arranged so that an amplitude of an output signal voltage is large and a distortion component thereof is small, that is, the S/N ratio is large. Also, a magnetic scale which can satisfy the above three requirements must be arranged so that a signal electric field is large, uniform and very close to a sine wave.
A magnetic sensor using magnetoresistance effect elements (hereinafter simply referred to as MR elements) which can be formed by a photolithography technique is proposed as the magnetic sensor which can satisfy the above requirements. However, even when the MR elements are employed, a reproduced output signal voltage is unavoidably decreased in accordance with the reduction of the grating pitch because a magnetic field formed by a magnetic scale is reduced. Accordingly, to increase the reproduced output signal voltage, a spacing between the magnetic scale and the magnetic sensor, i.e., so-called clearance must be reduced. By way of example, the clearance must be reduced to about 40 .mu.m. In addition, the clearance must be maintained as a constant value in order to keep the amplitude of the reproduced output signal voltage constant.
A material of a conventional magnetic scale is a magnetic alloy such as CuNiFe and FeCrCo. This magnetic alloy is partly not uniform in alloy component from a microscopic standpoint so that a uniform magnetic member, in other words, a magnetic field distribution corresponding to the sine wave having no distortion cannot be obtained over the full length of the magnetic scale. Further, a flatness of the surface of the above magnetic alloy is about 1s with the result that the clearance of about 40 .mu.m cannot be kept constant locally. As a consequence, the amplitude of a reproduced signal voltage cannot be kept constant.